Interest is growing in the use of sustainable and economical biological processes for generating materials of interest. Biological processes hold the promise of renewably using energy from the sun to make such materials. For example, energy from the sun can be stored in plant biomolecules such as the polysaccharides starch and cellulose. By fermentation of the simple sugars arising from breakdown of these polysaccharides, microbes can transfer the sun's energy into molecules of commercial interest to humans, including ethanol. Historically, large-scale polysaccharide breakdown has been accomplished by heat and chemicals, but in the past decades industrially produced starch hydrolytic enzymes have been employed to facilitate this process.
The tools of recombinant DNA technology arising in the 1980's have enabled the creation of transgenic organisms capable of expressing high levels of starch hydrolysis enzymes. In routine use today are alpha amylases, glucoamylases, and pullulanases, produced by recombinant microbes at the scale of tanker trucks per day. However, making biomolecules of interest by this process is lengthy and inherently inefficient. For example, energy is first transferred from the sun to plant polysaccharides, then from these plant polysaccharides to microbes that make starch hydrolysis enzymes, and then the enzymes thus produced are used to facilitate breakdown of additional plant polysaccharides used by yet another microbe to eventually form ethanol. Accordingly, using the same microbe that produces the material of interest to also produce the starch hydrolysis enzymes offers the opportunity for more efficient resource utilization (see for example, U.S. Pat. No. 5,422,267).
Such approaches have recently come to commercial fruition in the form of a glucoamylase-expressing yeast in the fuel ethanol industry. These approaches promise to reduce the use of expensive exogenously added enzymes. However, in this infant industry setting many unmet needs exist. One large need resides in engineering the biochemical pathways of a yeast host to support improved biochemical yield, e.g., ethanol yield.
Another need in the ethanol industry is to improve the levels of ethanol recovered in a yeast fermentation process. Glycerol produced by industrial yeast strains detracts from the potential yield of ethanol recovered. Yeast strains with partially or completely blocked glycerol biosynthesis have been described earlier, e.g., by Wang H-T et al. J. Bacteriol. 176 (22), 709 (1994); Eriksson P et al. Mol. Microbiol. 17 (1), 95, 1995; Björkqvist S et al. Appl. Environ. Microbiol. 63 (1), 128 (1997); Nissen T L et al. Yeast 16, 463 (2000); and Nevoigt E et al. Appl. Environ. Microbiol. 72 (8), 5266 (2006). All of these studies were conducted in haploid laboratory strains of the yeast Saccharomyces cerevisiae and are not necessarily directly applicable to industrial diploid/polyploid yeast strains. More recently, some publications report molecular engineering as an approach for industrial yeast strains with disrupted glycerol pathway. (See e.g., Guo Z-p et al. Appl. Microbiol. Biotechnol. 82, 287 (2009); Guo Z-p et al. Appl. Microbiol. Biotechnol. 38, 935 (2011);); Guo Z-p et al. Metabolic Engineering 13, 49 (2011)). However, in reality, these authors work with haploid derivatives of industrial yeast, which has different properties and are not industrial yeast strains themselves. As such, a need still exists for approaches to improve ethanol yield from industrial yeast strains.